In complex $Ce ^{4+} \rightarrow[ Xe ] 4 f ^{0} 5 d ^{0} 6 s ^{0}$

there is no unpaired electron so $\mu_{ m }=0$

$(b)$ ${ }_{64} Gd ^{3+} \rightarrow[ Xe ] 4 f ^{7} 5 d ^{0} 6 s ^{\circ}$

contain seven unpaired electrons so, $\mu_{ m }=\sqrt{7(7+2)}=\sqrt{63} B.M$

$(c)$ ${ }_{63} Eu ^{3+} \rightarrow\left[{ }_{54} Xe \right] 4 f ^{6} 5 d ^{0} 6 s ^{\circ}$

contain six unpaired electron

so, $\mu_{ m }=\sqrt{6(6+2)}=\sqrt{48} B . M$

Hence, order of spin only magnetic movement

$b > c > a$